This invention relates to a motion artefact rejection system for pulse oximeters; more particularly, it relates to a system for filtering out signals due to patient movement, i.e. motion artefact signals, from wanted signals.
The operation of pulse oximeters which measure arterial blood oxygen saturation and pulse rate is prejudiced when the patient performs any movement. Oximeters have difficulty in distinguishing the pulsating signals due to arterial blood flow from the pulsating signals due to patient movement. Since the results are calculated from this pulsatile signal and the size thereof, it is highly desirable to be able to distinguish signals from these two sources. The present invention, which encompasses an apparatus and the use thereof, reduces the severity of this problem and offers significant advantages to a clinician.
In general terms, a pulse oximeter apparatus will typically comprise the following units: a sensor, containing two LEDs of different wavelength (commonly 660 nm and 940 nm), and a photodetector, which are applied directly to a patient. The sensor is connected to the main instrument by a cable. The instrument contains a system to adjust LED power, hence controlling light intensity, and a system to analyse the incoming light from the photodetector. Means are provided to isolate the pulsatile components of these incoming light signals. The nonvarying ("DC signals") at each wavelength are either maintained equal by the LED power adjusting system, whereby the effects thereof cancel, or they may themselves be isolated and measured. The time-varying signals ("AC signals") then pass through an AGC (automatic gain control) system to ensure that they supply an adequate signal to an analogue-to-digital converter, where they are digitised. The AC and DC signals are then taken into a microprocessor, which analyses the AC signals for amplitude and frequency (corresponding to pulse rate). Oxygen saturation is calculated by the microprocessor by inserting the amplitudes of the various signals into the following formula: ##EQU1## and reading the result from an experimentally-determined reference table. The results may be displayed on LEDs or LCDs. There is additionally provided a system to judge whether motion artefact is present by examination of variability of AC signal frequency. If motion is judged to be present, displayed values are frozen and, if this state of affairs continues for any length of time, a warning message is given.
In use, the sensor is closely applied to a well-perfused region of a patient, such as a fingertip. Light from the LEDs needs to pass through a well-perfused region to ensure a good AC signal is obtained. The emergent light pulsates in intensity due to arterial pulsation. Since during systole the internal vessels are dilated, the total path length for the light is increased and intensity falls. Arterial blood is examined exclusively since it alone is the cause of the AC signals.
Patient movement interferes with the operation of pulse oximeters in several ways. If either the LEDs or photodetector is not fixed directly in contact with the skin, their distance from it may vary slightly when the patient moves. By simple 1/d.sup.2 function through air, measured light levels may change disastrously in real-life situations.
Additionally, even if the optical components are ideally fixed to the skin, the path length between them may change if the tissue is slightly deformed by the movement. Again, light level changes by this mechanism may seriously interfere with measurements. In this case, the function of intensity versus distance is more complicated than 1d/.sup.2, since, as tissue is deformed, its optical characteristics change. This is because of the mobility of the blood, the major absorbing species at the wavelengths in use; for instance, as the fingertip is compressed, the path length between the optical components will reduce, but, additionally, venous and capillary blood is squeezed out of the light path.
Furthermore, during severe motion, one or both optical transducers may be pulled laterally along the tissue under measurement, effectively changing the measurement site. This typically occurs when the cable connecting the sensor to the instrument is pulled and may cause major optical disturbance.
Since the AC signal is typically only 2-5% of the amplitude of the DC signal, it is this which is proportionally most seriously affected by movement artefact. Considering this, it is a reasonable approximation to apply a filtering algorithm to the AC signals and to ignore errors in the DC signals.
Surprisingly, it has now been discovered that the wanted AC signals, otherwise known as plethysmograph waveforms, have typical frequency versus power spectra as illustrated in accompanying FIG. 1. That is, about 90% of their energy is contained at the fundamental frequency (the pulse rate) with relatively little harmonic energy. Additionally, the unwanted signal caused by motion artefact frequently lies outside the frequency band of the pulse rate. Accompanying FIGS. 2 and 3 illustrate the frequency versus power spectra of signals with which motion artefacts, random and periodic, respectively, are interfering. It follows from these realisations that a bandpass filter may be adapted selectively to exclude motion artefact from wanted signals. Accompanying FIG. 4 illustrates the effectiveness of the present system in the removal of unwanted motion artefact signals from wanted plethysmograph signals.